Six-axis force sensor

ABSTRACT

A six-axis force sensor  1  consists of a sensor chip having a six-axis force sensor function formed using a semiconductor substrate  2  and film-forming technology. Connecting parts  5 A to  5 D are each made up of a high-rigidity bridge part and a low-rigidity elastic part. Semiconductor strain resistance devices Sya1-Sya3, Syb1-Syb3, Sxa1-Sxa3, and Sxb1-Sxb3 consisting of active layers are disposed on the front sides of the connecting parts. The elastic parts of the connecting parts absorb excess strains acting on the connecting parts and suppress the occurrence of strain spread over the whole semiconductor substrate. Because forces and moments of six specified axis components cause strains to occur selectively in corresponding strain resistance devices, by suitably combining measured results from selected resistance devices it is possible to effectively separate an applied external force into six components of force and moment and greatly suppress other axis interference in the measured results. As a results six components of force and moment of an external force can be measured with good reproducibility on the basis of the strains arising in the resistance devices disposed on the connecting parts.

FIELD OF THE INVENTION

[0001] This invention relates to a six-axis force sensor, andparticularly to a small and highly precise six-axis force sensor inwhich strain resistance devices fabricated using semiconductormanufacturing process technology are used and which can detect six forceand moment components and be utilized as a force-sensing sensor of arobot or the like.

BACKGROUND OF THE INVENTION

[0002] Modern automatic machines such as machine tools and robots, inthe course of their work or operation, perform work in which they applyforces to workpieces, or are themselves subject to actions of forcesfrom outside. Consequently, it is necessary for machine tools and robotsto detect forces and moments acting on them from outside and to performcontrol corresponding to these forces and moments. For controlcorresponding to external forces and moments to be carried out properly,it is necessary for the forces and moments acting from outside to bedetected exactly.

[0003] In this connection, various types of multi-axis force sensors tobe used as force-sensing sensors and man-machine interfaces have beenproposed. Generally, force sensors can be classified, according to thedetection method that they use, as either elastic-type force sensors,which detect a deformation proportional to a force, orforce-balance-type force sensors, which measure a force by balancing itwith a known force. As a principle structure, force sensors generallyhave multiple strain resistance devices provided on a distorting bodypart which deforms elastically in correspondence with external forces.

[0004] With this structure, when an external force acts on a distortingbody part of the multi-axis force sensor, electrical signalscorresponding to degrees of deformation of the distorting body part areoutputted from the strain resistance devices. On the basis of theseelectrical signals it is possible to detect two or more force or momentcomponents acting on the distorting body part.

[0005] To keep up with size reductions of devices equipped withmulti-axis force sensors, size reductions of multi-axis force sensorsthemselves are sought. Accordingly, there is an increasing need formulti-axis force sensors which have good sensitivity and high precisionwhile being small.

[0006] A typical multi-axis force sensor is the six-axis force sensor.The six-axis force sensor is a force sensor of the elastic typementioned above, and has multiple strain resistance devices on adistorting body part. The six-axis force sensor divides an externalforce into axial stress components (forces Fx, Fy, Fz) in the axisdirections and torque components (torques Mx, My, Mz) about the axisdirections of three orthogonal coordinate axes (an X-axis, a Y-axis, aZ-axis), and detects it as six axis components.

[0007] A first example of a multi-axis force sensor in related art isthe ‘Multiple Force Component Load Cell’ disclosed in JP-B-S.63-61609(Publication date Nov. 29, 1988; corresponding U.S. Patent: U.S. Pat.No. 4,448,083). This document discloses a six-axis force sensor. Thissix-axis force sensor has a construction wherein multiple strain gaugesare affixed to a distorting body having a solid (three-dimensional)structure. This sensor is a six-component force sensor and has astructure such that mutual interference arising among the six forcecomponents detected is reduced. The six-axis force sensor is made up ofa central force-receiving part, a fixed annular part around this, andbetween these, four T-shaped connecting parts equally spaced around theaxis of the force-receiving part. The strain gauges are affixed tolow-rigidity portions of the beams of the T-shaped connecting parts.

[0008] With this structure wherein strain gauges are affixed to adistorting body, size reduction is limited; manufacturingreproducibility is poor and dispersion arises among units; and alsoproblems such as peeling of the affixing layer arise due to repeatedshock stresses and thermal stresses. When peeling of the affixing layeroccurs, the measuring precision deteriorates. Alignment deviation alsocauses the measuring precision to deteriorate. The problem arises thatit is difficult to make the mounting positions accurate enough to ensuregood detection accuracy.

[0009] A second example of a multi-axis force sensor in related art isthe ‘Two-or-more-Component Force-Detecting Device’ disclosed in JapanesePatent Publication No. 2746298 (Published May 6, 1998, correspondingU.S. Patent: U.S. Pat. No. 4,884,083). In the multi-axis force sensordisclosed in this document, multiple strain resistance devices arefabricated on a substrate using semiconductor manufacturing processtechnology, and a strain gauge element is assembled integrally to adistorting body part. The substrate is made up of a peripheral part anda central part. According to this document, the problems of themulti-axis force sensor of the first related art mentioned above can beresolved, the precision of the fabrication process can be raised, thereproducibility of fabrication can be made good, and the multi-axisforce sensor can be reduced in size. However, with this sensor, there isa high probability of mutual interference arising among the six axiscomponents detected.

[0010] A third example of a multi-axis force sensor of related art isthe ‘Contact Force Sensor’ disclosed in JP-B-H.7-93445. In this contactforce sensor also, piezoelectric sensors made by forming resistancedevices on one side of an annular structural body made of asemiconductor are used, and semiconductor manufacturing processtechnology is utilized.

[0011] Of the multi-axis force sensors of these first through thirdexamples of related art, whereas in the first multi-axis force sensorstrain resistance devices (strain gauges) are affixed as externalelements, in the second and third multi-axis force sensors, strainresistance devices are formed integrally on a semiconductor substrate byutilizing semiconductor device manufacturing process technology. Thesecond and third multi-axis force sensors have the merit that they makeit possible to resolve the problems associated with the first multi-axisforce sensor.

[0012] However, related art multi-axis force sensors fabricated usingsemiconductor device manufacturing process technology have had thecharacteristic structurally that, when an attempt is made to detect aforce or moment on each of three orthogonal axes, the whole substratedistorts isotropically in correspondence with the applied force ormoment, and have also had the problem that the disposition of themultiple strain resistance devices on the substrate is not optimal andan external force acting on the distorting body part cannot be separatedinto components with good precision.

[0013] That is, in six-axis force sensors, there has been the problemthat for example when an external force is applied so that only an axialstress component Fx arises, stresses arise and outputs are produced inconnection with components other than Fx, which should properly be 0.There has been the problem that it is difficult to separate an externalforce applied from an unknown direction into individual components withgood precision. The electrical signal components outputted from theresistance devices corresponding to the respective axis componentssuperpose onto the other axes, and the measuring sensitivity of the axiscomponents of force or moment decreases.

[0014] The problem of not being able to separate the axis components(forces and moments) of an external force acting on the distorting bodypart in a six-axis force sensor is known as the problem of other axisinterference. This problem of other axis interference is one whichcannot be ignored from the point of view of realizing a practicalsix-axis force sensor.

[0015] The problem of other axis interference in a six-axis force sensorwill now be explained more specifically, from a mathematical point ofview, using equations.

[0016] In a six-axis force sensor, as mentioned above, as six axiscomponents pertaining respectively to an X-axis, a Y-axis and a z-axis,forces Fx, Fy and Fz and moments Mx, My and Mz are detected. Thesix-axis force sensor outputs six signals Sig1 to Sig6 (‘computedresistance change proportions’) using resistance changes of strainresistance devices provided on a distorting body part and on the basisof an external computing part. These six output signals Sig1 to Sig6 areassociated with the six axis components Fx, Fy, Fz, Mx, My, Mz using 6×6matrix elements obtained by finding in advance the size (electricalchange proportion) of each signal with respect to an input made byapplying as an external force a single axis component only.

[0017] For a six-axis force sensor, the six axis components Fx, Fy, Fz,Mx, My, Mz will be written respectively as F1, F2, F3, F4, F5, F6(generally, ‘Fi:i=1-6’). The above-mentioned six output signals Sig1 toSig6 will be written S1, S2, S3, S4, S5, S6 (generally, ‘Si:i=1-6’).

[0018] Between Si and Fi above, expressed with a matrix equation (thesymbols ‘( )’ in the equation indicating matrices), the followingrelationship holds.

(Si)=(mij)×(Fj) (j=1-6)  (101)

[0019] That is, the equation (101) has the following meaning:

[0020] S1=m11.F1+m12.F2+m13.F3+m14.F4+m15.F5+m16.F6

[0021] S2=m21.F1+m22.F2+m23.F3+m24.F4+m25.F5+m26.F6

[0022] . . . (abbreviated)

[0023] S6=m61.F1+m62.F2+m63.F3+m64.F4+m65.F5+m66.F6

[0024] In equation (101), by finding in advance the computed respectiveresistance change proportions S1-S6 corresponding to the input of asingle component only, it is possible to obtain the matrix elements mijof the matrix (mij). By calculating the inverse matrix (mij)⁻¹ of theobtained matrix (mij), the following equation is obtained:

(Fi)=(mij)⁻¹×(Sj)=(m′ij)×(Sj)  (102)

[0025] The equation (102) has the following meaning:

[0026] F1=m′11.S1+m′12.S2+m′13.S3+m′14.S4+m′15.S5+m′16.S6

[0027] F2=m′21.S1+m′22.S2+m′23.S3+m′24.S4+m′25.S5+m′26.S6

[0028] . . . (abbreviated)

[0029] F6=m′61.S1+m′62.S2+m′63.S3+m′64.S4+m′65.S5+m′66.S6

[0030] In the above equation (102), “m′ij” is a matrix element of the6×6 inverse matrix (mij)⁻¹.

[0031] From the above equation (102), on the basis of the computedresistance change proportions (S1-S6) obtained from the resistancechange proportions of the semiconductor strain resistance devices, it ispossible to calculate the six axis components F1-F6 (the forces andmoments of each axis direction).

[0032] In equations (101) and (102) above, if the values of the matrixelements mij, m′ij are all large, then for example when the computedresistance change proportions Si fluctuate due to the superposition ofnoise, that influence appears in the measurement values of F1-F6. Thatis, when there is an input consisting of a single component only as anexternal force, although the inputs of the other components are “0”,there is a high probability of the phenomenon arising of the measurementresults not being “0” due to disturbances such as noise.

[0033] AS mentioned above, the obtained measured value of one of the sixaxis components, that is, forces or moments, fluctuating as a result ofa force or moment of another axis is defined as ‘other axis interferenceoccurring’.

[0034] Ideally, in the matrix (m′ij), the non-diagonal elements i.e. theelements other than the diagonal elements m′11, m′22, m′33, m′44, m′55and m′66, should be “0”. In this case, the above-mentioned equation(102) becomes as follows:

[0035] F1=m′11.S1

[0036] F2=m′22.S2

[0037] . . . (abbreviated)

[0038] F6=m′66.S6

[0039] It this relationship holds, the calculation becomes extremelysimple, and other axis interference can be prevented.

[0040] In practice, even if the non-diagonal elements cannot be made“0”, if the values of the non-diagonal elements can be made extremelysmall compared to the diagonal elements, the problem of other axisinterference can be reduced.

[0041] However, with the second and third multi-axis force sensors ofrelated art mentioned above, because the whole substrate distorts andinsufficient consideration has been given to the suitability of thedisposition pattern of the semiconductor strain resistance devices, thenon-diagonal elements of the matrix (m′ij) cannot be made “0” or madesufficiently small compared with the diagonal elements, and theprobability of other axis interference occurring is high. Also, with themulti-axis force sensors mentioned above, due to other axis interferencereadily occurring, noise caused by unexpected disturbances and the likesuperposes on the electrical signals from the strain resistance devices,and consequently there is a high risk of measurement results fluctuatinggreatly with other axis interference as the cause. Consequently, whenthe second and third multi-axis force sensors mentioned above are usedon a robot or the like, depending on their installation conditions,other axis interference causes their measured values to fluctuate, andwhen they are made general-purpose parts there are problems withreproducibility and robustness.

[0042] A fourth example of a multi-axis force sensor of related art isthe ‘Micro-Manipulator Having Force Sensor’ disclosed in JapaneseUnexamined Patent Publication No. H.11-333765. The force sensordisclosed in this document is fabricated using semiconductormanufacturing process technology, like the second and third related artexamples mentioned above, and a three-component force sensor made up ofa base and a central thick part and thin supporting parts connectingthereto and having strain sensors provided on the supporting part isshown.

[0043] To resolve the above-mentioned shortcomings of the multi-axisforce sensors of the aforementioned second and third related artexamples using semiconductor manufacturing process technology, in thisfourth related art example, a structure is proposed wherein componentsof strain are separated axis by axis. However, although this structureachieves a slight improvement compared to other related art, it is aconstruction for performing detection of forces in the directions ofthree axes (X-axis, Y-axis, Z-axis), and when it is used as a six-axisforce sensor its component-separating capability is inadequate and itcannot resolve the problem of other axis interference.

[0044] A further, fifth example of a multi-axis force sensor of relatedart is the ‘Sensor’ disclosed in JP-B-H.5-75055. This sensor is formedusing a semiconductor substrate and has a central supporting body, aperipheral supporting body, and a plurality of connecting parts (beams)connecting these. According to FIG. 1 of this document, a resistancefilm pattern made up of multiple resistance devices is formed integrallywith the surfaces of two predetermined connecting parts (beams) byfilm-forming technology. Because the connecting parts are parts of thesemiconductor substrate, they have a thin plate shape. When the sensorreceives a force on the central supporting body, the semiconductorsubstrate itself bends as a whole, and six force components are takenout by the multiple resistance devices provided on the connecting parts.In the sensor of this related art example also, there is a possibilityof mutual interference among the detected six force components.

[0045] Another issue addressed by the invention will now be discussed. Asix-axis force sensor fabricated using semiconductor manufacturingprocess technology contributes to sensor device size reduction. To makea six-axis force sensor small, the semiconductor substrate becomes smalland becomes thin. In a six-axis force sensor formed using asemiconductor substrate, the semiconductor substrate itself functions asa distorting body. Consequently, there is a limit on the range of forceswhich can be measured, which depends on the basic strength of thesemiconductor substrate. From the point of view of practicalapplication, there is a need for a sensor to be designed so that thislimit is not problematic, and for the measurement range to be raised towiden the range of applications.

SUMMARY OF THE INVENTION

[0046] It is therefore an object of the invention to solve the problemsmentioned above and provide a six-axis force sensor with which indetecting an applied external force it is possible to suppress otheraxis interference, detect axis components of force and moment with highprecision, and increase robustness and reproducibility.

[0047] It is another object of the invention to provide a six-axis forcesensor which while suppressing other axis interference dramaticallyraises the detectable level of force, has a widened range ofapplications, and contributes to practical usability of six-axis forcesensors.

[0048] To achieve these objects, a six-axis force sensor according tothe invention has the following construction.

[0049] The six-axis force sensor according to the invention consists ofa thin plate-shaped sensor chip formed using a substrate bysemiconductor film-forming processes and having a six-axis force sensorfunction. The sensor chip has an action part for an external force to beapplied to, a support part to be fixed to an external structure, and anumber of connecting parts each having a high-rigidity bridge partjoined to the action part and a low-rigidity elastic part joined to thesupport part and connecting together the action part and the supportpart. A number of strain resistance devices consisting of active layersare formed on at least one of the front and rear faces of each of theconnecting parts in an area where deformation strain effectively occurs,and these strain resistance devices are electrically connected tocorresponding electrodes provided on the support part.

[0050] The six-axis force sensor is a sensor chip having a six-axisforce sensor function formed using a semiconductor substrate andfilm-forming technology. The connecting parts are each made up of ahigh-rigidity bridge part and a low-rigidity elastic part. Semiconductorstrain resistance devices (Sya1-Sya3, Syb1-Syb3, Sxa1-Sxa3, Sxb1-Sxb3)consisting of active layers are disposed on the front sides of theconnecting parts. The elastic i.e. low-rigidity parts of the connectingparts have the function of absorbing excess strains acting on theconnecting parts and suppressing the occurrence of strains extendingover the semiconductor substrate as a whole. Because forces and momentsof specified axis components cause strains to occur selectively incorresponding strain resistance devices, by suitably combining measuredresults from selected resistance devices it is possible to effectivelyseparate an applied external force into six components of force andmoment and greatly suppress other axis interference in the measuredresults. As a result, a six-axis force sensor according to the inventioncan measure six components of force and moment of an external forceapplied to the action part with good reproducibility on the basis of thestrains arising in the resistance devices disposed on the connectingparts

[0051] In the six-axis force sensor, the connecting parts are disposedwith uniform spacing around the action part. The low-rigidity parts areelastic parts each connected to the support part at two locations. Thehigh-rigidity parts are bridge parts connected to the action part.Accordingly, the elastic parts have the function of absorbing excessstrains acting on the bridge parts and suppressing the occurrence ofstrains extending over the semiconductor substrate as a whole caused bythe application of a force or moment in one direction, and because aforce or moment in a given direction causes a strain selectively in acorresponding resistance device, other axis interference is greatlysuppressed.

[0052] Preferably, the connecting parts are disposed with uniformspacing around the action part and so that adjacent connecting parts aremutually perpendicular.

[0053] Preferably, the action part is square and each of the connectingparts is formed in a T-shape made by its elastic part and its bridgepart and is disposed at a respective one of the four sides of the actionpart. The strain resistance devices are disposed on an area of thesurface of the bridge part near the boundary between the bridge part andthe action part.

[0054] Preferably, the strain resistance devices are disposed on anarrowed portion formed in the bridge part and, more preferably, aredisposed on the bridge part of the respective connecting part inparallel with the length direction of the bridge part and arranged sideby side in a line in a direction perpendicular to the length directionof the bridge part. Resistance devices for temperature compensation canalso be disposed on the support part.

[0055] In a six-axis force sensor according to the invention asdescribed above, the strain resistance devices are disposed on the surface of the bridge part in an area where, when an external force isapplied to the action part, greater strains arise than in the actionpart and other areas of the connecting part. As a result, the resistancedevices can detect required components of force and moment selectively,the resistance changes arising in the resistance devices with respect tothe external force can be made large, the precision with which theforces and moments of an external force applied to the action part aremeasured increases, and robustness and stability can be obtained in themeasurement results.

[0056] Because the strain resistance devices are disposed on the surfaceof each of the connecting parts near the joining portion between itsbridge part and the action part, they are disposed in the area wherestrains caused by the external force most concentrate. As a result, thechanges in the resistance values of the semiconductor resistance devicesarising from an external force can be made large.

[0057] If the resistance devices are disposed on a narrowed portionformed in the bridge part, because in the connecting parts correspondingto components of force and moment of an external force applied to theaction part the greatest stresses arise in the narrowed part, strainsconcentrate most in this narrowed part, and the resistance changes ofthe resistance devices caused by the external force can be made large.

[0058] Disposing multiple resistance devices on each bridge part inparallel with the long axis direction of the bridge part makes itpossible to combine resistance change proportions obtained from multipleresistance devices as electrical signals are taken out. This makes itpossible to select resistance devices so that other axis interference isprevented, i.e. so that resistance change proportions pertaining toforces and moments in other than a certain direction cancel each otherout, and makes it possible to compose optimal formulas for obtaining sixcomponents of force and moment from resistance change proportions ofthese resistance devices. On the basis of these formulas, thenon-diagonal elements in a matrix expressing the relationship betweenforces and moments and resistance change proportions can be made “0” oramply small.

[0059] By disposing semiconductor resistance devices for temperaturecompensation on the support part and interconnecting them tocorresponding electrodes it is possible to detect resistance changes dueto the temperature of the surroundings. By correcting measured resultsof resistance change in correspondence with the temperaturecharacteristics of the semiconductor resistance devices on this basis itis possible to obtain force and moment measurements unaffected by theambient temperature.

[0060] In the six-axis force sensor as arranged above, preferably, aguard interconnection at a ground potential is provided so as tosurround the non-ground interconnections for the strain resistancedevices. In this case, in the detection of currents from the resistancedevices, disturbances caused by high-frequency noise are suppressed, andbecause the guard interconnection shields against cross-talk from theinterconnections of other resistance devices, the S/N-ratio of currentmeasurement of resistance changes of the resistance devices can beincreased, the precision with which component forces and moments of anapplied external forces are measured can be increased, and robustnessand stability are obtained in the measurement results.

[0061] In the six-axis force sensor as arranged above, preferably, abias electrode for impressing a bias voltage on the substrate isprovided. With this construction, by means of an impressed bias, it ispossible to grow a depletion layer at the active layer interfaces andthereby to effect insulation between the active layers and thesemiconductor substrate and between adjacent active layers, andconsequently leak currents decrease and current noise influences can bereduced. Also, by electrically fixing the substrate at a fixedpotential, variation of potential can be prevented and resistance tonoise can be increased, and resistance changes in the semiconductorresistance devices can be measured with high precision; consequently,the measurement precision of component forces and moments of an appliedexternal force can be increased, and robustness and stability can beobtained in the measurement results.

[0062] Preferably, at least one of the strain resistance devices on eachof the connecting parts is disposed on the rear side of the substrate inthe area corresponding to the area on the front side of the substratewhere the strain resistance devices are disposed on the bridge part. Bymeans of this construction, on the basis of resistance changes ofresistance devices formed on both the front side and the rear side ofthe substrate, an external force applied to the action part can bebetter resolved into specific force and moment components compared towhen resistance devices are provided only on the front side of thesubstrate, and more robustness and stability can be obtained in themeasurement results.

[0063] In this six-axis force sensor, the internal corners at thejoining portions of the connecting parts and the action part, thejoining portions of the connecting parts and the support part, and thejoining portions of the elastic parts and the bridge parts, are allworked to a circular arc shape. With this construction, stresses at thejoining portions are dispersed, concentration of stress at the internalcorners arising from the external force applied to the action part issuppressed, structural strength is increased, and the measurable rangeof applied external forces can be widened.

[0064] Another six-axis force sensor provided by the invention has athin plate-shaped sensor chip formed using a substrate by semiconductorfilm-forming processes and having a six-axis force sensor function andincluding at least an action part for receiving an external force and asupport part supporting the action part, and a structural body providedaround the sensor chip and including an external force application partto which an external force is applied, a plinth part for supporting thesensor chip, an external force buffering mechanism fixing the externalforce application part to the plinth part, and an external forcetransmitting part, and the external force application part and theaction part are linked by the external force transmitting mechanism.

[0065] In this six-axis force sensor, a sensor chip made using asemiconductor substrate is provided with a structural body made up of anexternal force application part, a plinth part, an external forcebuffering mechanism and an external force transmitting mechanism, sothat the external force is attenuated as it acts on the sensor chip. Bythis means it is possible, while suppressing the problem of other axisinterference in a six-axis force sensor, to raise the load-withstandingcapability of the sensor, raise its dynamic range, enlarge itsmeasurable range of external forces, and raise its practical usability.Also, a buffering mechanism for the six-axis force sensor is realizedwith a simple structure, and by optimizing this structure it is possibleto adjust the load-withstanding characteristics of the sensor. It ispossible to manufacture a six-axis force sensor which is resistant toexternal noise, has high reproducibility of detection performance, canbe fabricated without dispersion arising in its performance, and isoptimized with respect to one of various force ranges.

[0066] In the construction described above, preferably, the sensor chiphas an action part, a support part, four connecting parts, and multiplestrain resistance devices fabricated by semiconductor film-formingprocesses on parts where deformation will occur. An insulating membercan be provided between the sensor chip and the plinth part.

[0067] Because the sensor chip has four connecting parts and multiplestrain resistance devices in a predetermined disposition pattern, thissix-axis force sensor has a detection performance such that other axisinterference is suppressed as far as possible. Because an insulatingmember is interposed between the sensor chip and the plinth part,electrical noise can be prevented from passing from the structural bodyhaving the buffering mechanism to the sensor chip.

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The above-mentioned objects and characteristics of the inventionwill become clearer from the following description of presentlypreferred embodiments thereof illustrated in the accompanying drawings,wherein:

[0069]FIG. 1 is a plan view showing the structure of a first preferredembodiment (sensor chip) of a six-axis force sensor according to theinvention;

[0070]FIG. 2 is an enlarged detail view of a connecting part (5C) of thesix-axis force sensor of the first preferred embodiment;

[0071] FIGS. 3(a) through (e) are sectional views of the six-axis forcesensor of the first preferred embodiment at different stages in amanufacturing process;

[0072]FIG. 4 is a perspective view illustrating forces and moments ofdifferent axes acting on an action part of the six-axis force sensor ofthe first preferred embodiment;

[0073]FIG. 5 is a plan layout view showing positions of strainresistance devices formed on the surfaces of connecting parts (5A-5C) ofthe six-axis force sensor of the first preferred embodiment;

[0074] FIGS. 6(a) and (b) are a plan view and a sectional view showing astate of deformation arising when a force −Fy is applied in a Y-axisdirection to the action part of the six-axis force sensor (sensor chip)of the first preferred embodiment;

[0075] FIGS. 7(a) and (b) are similar views showing a state ofdeformation arising when a force Fx is applied in an X-axis direction tothe action part of the six-axis force sensor (sensor chip) of the firstpreferred embodiment;

[0076] FIGS. 8(a) and (b) are similar views showing a state ofdeformation arising when a force −Fz is applied in a Z-axis direction tothe action part of the six-axis force sensor (sensor chip) of the firstpreferred embodiment;

[0077] FIGS. 9(a) and (b) are similar views showing a state ofdeformation arising when a moment My is applied about the Y-axisdirection to the action part of the six-axis force sensor (sensor chip)of the first preferred embodiment;

[0078] FIGS. 10(a) and (b) are similar views showing a state ofdeformation arising when a moment Mx is applied about the X-axisdirection to the action part of the six-axis force sensor (sensor chip)of the first preferred embodiment;

[0079] FIGS. 11(a) and (b) are similar views showing a state ofdeformation arising when a moment Mz is applied about the Z-axisdirection to the action part of the six-axis force sensor (sensor chip)of the first preferred embodiment;

[0080] FIGS. 12(a), (b) and (c) are dimension views showing the shapesand dimensions of constituent parts of a specific example of a prototypesix-axis force sensor;

[0081]FIG. 13 is a table showing a relationship between force/momentapplied to the action part in different axis directions and resultingcomputed resistance change proportion per unit of the appliedforce/moment in the six-axis force sensor of the first preferredembodiment;

[0082]FIG. 14 is a rear side view showing positions of strain resistancedevices formed on the rear side of a semiconductor substrateconstituting the six-axis force sensor of the first preferredembodiment;

[0083]FIG. 15 is a table showing a relationship between force/momentapplied to the action part in different axis directions and resultingcomputed resistance change proportion per unit of the appliedforce/moment in the six-axis force sensor of the first preferredembodiment when four strain resistance devices are provided on its rearside;

[0084]FIG. 16 is a table showing a relationship between force/momentapplied to the action part in different axis directions and resultingcomputed resistance change proportion per unit of the force/moment inthe six-axis force sensor of the first preferred embodiment when twelvestrain resistance devices are provided on its rear side;

[0085]FIG. 17 is a plan view showing a second preferred embodiment of asix-axis force sensor according to the invention;

[0086]FIG. 18 is a side view showing schematically a third preferredembodiment of a six-axis force sensor according to the invention;

[0087]FIG. 19 is a table showing a matrix relating applied axisforce/moments Fx, Fy, Fz, Mx, My, Mz to resulting output signalsSig1-Sig6 in the six-axis force sensor of the third preferredembodiment;

[0088]FIG. 20 is a perspective view showing a first specific structureof a six-axis force sensor according to the third preferred embodiment;

[0089]FIG. 21 is a perspective view from below showing the positions ofa plate-like frame member, a sensor chip and an external forceapplication part of the six-axis force sensor of the first specificexample;

[0090]FIG. 22 is a perspective view from above showing the positions ofthe plate-like frame member and the sensor chip in the six-axis forcesensor of the first specific example;

[0091] FIGS. 23(a) and (b) are graphs showing for comparison ameasurement characteristic example (a) pertaining to the six-axis forcesensor of the first specific example and a measurement characteristicexample (b) pertaining to the sensor chip on its own;

[0092]FIG. 24 is a perspective view showing a second specific structureof the six-axis force sensor of the third preferred embodiment;

[0093]FIG. 25 is a perspective view showing a third specific structureof the six-axis force sensor of the third preferred embodiment; and

[0094]FIG. 26 is a perspective view showing a fourth specific structureof the six-axis force sensor of the third preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0095] Preferred embodiments of the invention will now be described,with reference to the accompanying drawings.

[0096] First, with reference to FIG. 1, a first preferred embodiment ofa six-axis force sensor according to the invention will be described. InFIG. 1, the six-axis force sensor 1 of this preferred embodiment isformed using a semiconductor substrate 2 which is preferably square inplan shape. The six-axis force sensor 1 has a plate-like shape derivedfrom the semiconductor substrate. The six-axis force sensor 1 ispreferably made by applying semiconductor manufacturing processtechnology (etching processes such as photolithography, resistpatterning, ion implantation, film-forming processes such as P-CVD,sputtering, RIE) to one of its surfaces, to work the plan shape of thesquare semiconductor substrate itself and to carry out film-forming inpredetermined regions of one surface of the semiconductor substrate. Inthis way, the six-axis force sensor 1 of this first preferred embodimentis formed as a semiconductor sensor device, and from this point of viewit will hereinafter also be called ‘the sensor chip 1’.

[0097] In the sensor chip 1, a functional part for detecting six axiscomponents as a six-axis force sensor is made up of twelve strainresistance devices (piezo-resistance devices; hereinafter called‘resistance devices’) Sxa1-Sxa3, Sxb1-Sxb3, Sya1-Sya3, Syb1-Syb3,consisting of an active layer (diffusion layer) formed on one surface ofthe semiconductor substrate 2.

[0098] The total of twelve resistance devices are disposed in sets ofthree (Sxa1, Sxa2, Sxa3), (Sxb1, Sxb2, Sxb3), (Sya1, Sya2, Sya3), (Syb1,Syb2, Syb3), each set being disposed on one of four T-shaped connectingparts 5A-5D, which will be further discussed later, along a boundarybetween the respective connecting part and an action part 4.

[0099] As shown clearly in FIG. 1, FIG. 4 and FIG. 5, the semiconductorsubstrate 2 has holes A, B, C, D, X, L, M and N formed passing throughit in the thickness direction of the substrate. The internal corners ofthe holes A, B, C, D, K, L, M and N are each worked to a circular arcshape. The sensor chip 1 is functionally divided by the holes A, B, C,D, K, L, M and N into multiple regions. The sensor chip 1 is made up ofthe action part 4, which is centrally positioned and substantiallysquare in shape; a square frame-shaped support part 3, positioned aroundthe action part 4; and the four T-shaped connecting parts 5A, 5B, 5C and5D, which are positioned between the action part 4 and the support part3 at their respective four side parts and connect the two together. Anoutside force is applied to the action part 4. The four connecting parts5A to 5D each form a T-bridge, respectively having a bridge part 5Aa andan elastic part 5Ab, a bridge part 5Ba and an elastic part 5Bb, a bridgepart 5Ca and an elastic part 5Cb, and a bridge part 5Da and an elasticpart 5Db. The elastic parts 5Ab, 5Bb, 5Cb and 5Db of the connectingparts 5A to 5D border the regions A, B, C, D respectively and are eachconnected to the support part 3 at both of their ends. The bridge parts5Aa, 5Ba, 5Ca and 5Da of the connecting parts 5A to 5D each have one end(the inner end) connected to one side of the substantially square actionpart 4 and the other end (the outer end) connected to the respectiveelastic part. The connecting parts, each made up of a bridge part and anelastic part; the joining portions between the connecting parts and theaction part; and the joining portions between the connecting parts andthe support part, are formed integrally as the semiconductor substrate2.

[0100] The joining portions between the bridge parts 5Aa, 5Ba, 5Ca and5Da, the elastic parts 5Ab, 5Bb, 5Cb and 5Db and the action part 4 areeach worked, preferably R-worked, to a circular arc shape, to distributestress caused by the external force applied to the action part 4 andthereby give them strength with respect to the applied external force.Specifically, for example the parts having an internal 90° angle areR-worked to R0.1 (radius 0.1 mm)

[0101] In the example shown in FIG. 1, in the semiconductor substrate 2the connecting parts 5 are formed into a T-shape (T-shaped beam) by theholes A, B, C, D, K, L, M and N. However, as long as the requiredelastic function is fulfilled, the connecting parts 5 can be made someother shape, for example a Y-shape or the like.

[0102] The resistance devices Sya1, Sya2 and Sya3 are formed on theconnecting part 5A near the joining portion between the action part 4and the bridge part 5Aa. That is, they are disposed on the surface ofthe part of the connecting part 5A where strain caused by the externalforce applied to the action part 4 arises the most. The resistancedevices Sya1, Sya2 and Sya3 are formed so that they line up in the widthdirection of the bridge part 5Aa and the long axis direction of each isparallel with the long axis direction of the bridge part 5Aa.

[0103] The other resistance devices Syb1-Syb3, Sxa1-Sxa3 and Sxb1-Sxb3are formed respectively near the joining portions between the actionpart 4 and the bridge part 5Ca, the action part 4 and the bridge part5Ba, and the action part 4 and the bridge part 5Da, similarly to theresistance devices Sya1-Sya3.

[0104] As shown in FIG. 2, resistance devices Syb1 to Syb3 eachconsisting of an active layer S extending in the long axis direction ofthe bridge part 5Ca are formed near the joining portion between theconnecting part 5C and the action part 4. In the positionalrelationships of the resistance devices Syb1 to Syb3, the resistancedevice Syb2 is disposed in the center (on the centerline of the actionpart 4) of the bridge part 5Ca, and the resistance devices Syb1 and Syb3positioned on either side of it are disposed in positions symmetricalabout the resistance device Syb2 at the bridge part 5Ca.

[0105] Here, to form the resistance devices compactly in a small area,the active layer S forming each of the resistance devices is formeddoubling back on itself several times so that its overall length islong. However, the shape of the resistance devices is not limited to theshape shown in the drawings.

[0106] Also, narrowed parts 6C can be formed, by removing the portionsshown with dashed lines in FIG. 2. That is, with the purpose ofconcentrating the strain caused by the application of the externalforce, narrowed parts 6C, 6A, 6B and 6D can be formed in the bridge part5Ca and the other bridge parts 5Aa, 5Ba and 5Da. Although these narrowedparts may be formed anywhere on the bridge parts, it is desirable thatthey be formed at the joining portions between the bridge parts and theaction part 4.

[0107] When narrowed parts 6C are formed as shown in FIG. 2, theresistance devices Syb1 to Syb3 are each made by forming a respectiveactive layer S in the area between the narrowed parts 6C. Also,irrespective of whether narrowed parts 6C are provided or not, theactive layers S each have one end connected by a contact CN1 to a GND(ground) interconnection 7 as resistance device and the other endconnected by a contact CN2 to a signal interconnection 8.

[0108] The GND interconnections 7 are formed so as to lie between thesignal interconnections 8 leading from the resistance devices Syb1, Syb2and Syb3 and so as to also surround the respective active layers S ofthe resistance devices Syb1, Syb2 and Syb3. As a result of theresistance devices and the signal interconnections 8 being isolated bymeans of the GND interconnections 7 like this, in the detection ofcurrents from the resistance devices, the GND interconnections 7 preventdisturbances caused by high-frequency noise in the environment. Also,because this isolating structure shields against crosstalk noise fromother resistance devices and their signal interconnections, it makes itpossible to raise the S/N ratio of current measurement of the resistancechanges arising from piezoelectric effect in the resistance devices.

[0109] Although in this preferred embodiment one end of each resistancedevice is connected to a ground potential, depending on the measurementmethod it may be that the resistance devices are not connected to aground potential. In this case, it is necessary for a GNDinterconnection dedicated for use as a guard to be provided separatelyfrom the signal lines of the resistance devices and for this GNDinterconnection to be connected to a ground potential independently.

[0110] Returning now to FIG. 1, this FIG. 1 will be explained further.The multiple GND interconnections 7 are connected to GND electrodes 9formed on the surface of the support part 3. The GND electrodes 9 areconnected to a GND interconnection 11 formed on the surface of theperiphery of the support part 3. A ground potential from an externalpower supply (not shown) is impressed on the GND electrodes 9 and theGND interconnection 11.

[0111] The signal interconnections 8 are severally connected to signalelectrodes 10 formed on the surface of the periphery of the support part3. The signal electrodes 10 are electrodes for performing measurement ofthe resistance values of the resistance devices and are connected to anoutside measuring instrument or device for analyzing the appliedexternal force (not shown).This measuring instrument or analyzing devicemeasures the resistance values from current-voltage characteristics andobtains resistance change proportions of the resistance devicesresulting from the applied force.

[0112] A bias electrode 12 is an electrode for impressing a bias voltageon the semiconductor substrate 2 and is supplied with a predeterminedvoltage from the external power supply. A bias voltage impressed by wayof this bias electrode 12 grows a depletion layer at the interfaces withthe active layers S. This depletion layer effects insulation between theactive layers S and the semiconductor substrate 2 and between adjacentactive layers S and so prevents leakage currents arising there andreduces influences of current noise. Also, keeping the semiconductorsubstrate 2 electrically at a fixed potential prevents potentialdispersion and improves noise tolerance and makes it possible forresistance changes based on piezoelectric effect corresponding tostrains of the resistance devices formed on the bridge parts 5Aa, 5Ba,5Ca, 5Da to be measured with high precision.

[0113] Resistance devices 13 are provided to effect temperaturecompensation. One end of each of the resistance devices 13 is connectedby an interconnection to the GND interconnection 11, and the other endis connected by an interconnection to an electrode 14. A predeterminedvoltage is impressed on the electrodes 14, and on the basis of thecurrent flowing here the ratio of the resistance value of the resistancedevice 13 to its resistance value at room temperature is obtained. Bymeans of this resistance value ratio and on the basis of the ambienttemperature, compensation of the resistance values of the resistancedevices formed on the bridge parts 5Aa, 5Ba, 5Ca, 5Da is carried out.That is, by correcting the measured results of resistance change of theresistance devices for external force measurement on the basis of theresistance change of the resistance device 13, which is not subject tothe influence of the external force, it is possible to measure forcesand moments without the results being influenced by the surroundingtemperature.

[0114] As described above, the sensor chip or six-axis force sensor 1 ismade up of an action part 4 to which an external force is applied, asupport part 3 fixed to an external part, and four T-shaped connectingparts 5A, 5B, 5C and 5D disposed around the action part 4. Theconnecting parts 5A, 5B, 5C and 5D are made up of elastic parts 5Ab,5Bb, 5Cb and 5Db, each connected at two locations to the support part 3,and bridge parts 5Aa, 5Ba, 5Ca and 5Da, each connected to the actionpart 4.

[0115] In the six-axis force sensor 1, when strains arise in theresistance devices formed on the bridge parts 5Aa, 5Ba, 5Ca, 5Da due toan external force applied to the action part 4, the elastic parts 5Ab,5Bb, 5Cb and 5Db prevent the occurrence of strain over the wholesemiconductor substrate 2 caused by this external force on the basis ofthe relationships of forces acting between the action part 4 and thebridge parts 5Aa, 5Ba, 5Ca, 5Da. Consequently, with this six-axis forcesensor 1, a selective strain pertaining to a force or moment in aspecified direction can be made to arise in each of the resistancedevices, and an external force applied to the action part 4 can beseparated into individual force and moment components.

[0116] Next, an example of a method for manufacturing the sensor chip 1will be described, with reference to FIGS. 3(a) through (e). In FIG.3(a), boron, a p-type impurity, is ion-implanted into a semiconductorsubstrate 2 consisting of n-type (100) silicon using as a mask a resistpattern (not shown) for resistance device formation formed byphotolithography. The resistance devices formed on the surface of thesemiconductor substrate 2 are the above-mentioned strain resistancedevices Sya1-Sya3, Syb1-Syb3, Sxa1-Sxa3, Sxb1-Sxb3 and the resistancedevices 13.

[0117] The resist pattern is then removed, and by p-CVD (plasma ChemicalVapor Deposition) a silicon oxide film is grown as an interlayerinsulation film 20. Then, by heating the semiconductor substrate 2, theimplanted boron ions are activated to form the active layers S. Here,for example the thickness of the semiconductor substrate 2 is 525 μm andthe thickness of the interlayer insulation film 20 is 300 nm. Theresistance value of the resistance devices is 53 Ω.

[0118] Referring now to FIG. 3(b), a resist pattern (not shown) forforming contact holes is formed, and with this resist pattern as a maskthe interlayer insulation film 20 where contact holes are to be formedis removed using BHF (Buffered Hydrofluoric acid). The resist pattern isthen removed, Al—Si (an alloy of aluminum and silicon) is sputtered to athickness of about 1 μm over the entire surface of the semiconductorsubstrate 2, and a heat-treatment for forming an ohmic junction iscarried out.

[0119] In this way, a contact CN between a bias electrode 12 and thesemiconductor substrate 2, and contacts CN2 (and the contacts CN1 shownin FIG. 2) of the active layers S are formed.

[0120] Next, in FIG. 3(c), a resist pattern (not shown) for forming theGND interconnections 7 and 11, the signal interconnections 8, the otherinterconnections, and the electrodes 9, 10, 12 and 14 is formed byphotolithography, and unnecessary areas of metal are removed by wetetching to pattern the interconnections and electrodes. The resistpattern is then removed, and a SiN film (silicon nitride film) is formedby p-CVD as a passivation film.

[0121] A resist pattern (not shown) for bonding pad formation is thenformed, and the SiN film in areas to become openings for the electrodes9, 10, 12 and 14 is removed.

[0122] Next, as shown in FIG. 3(d), a resist pattern R1 to form theholes A, B, C, D, X, L, M and N shown in FIG. 1 and to facilitate sensorchip separation is formed. Then, using wax, the semiconductor substrate2 is affixed to a dummy wafer 21.

[0123] Referring now to FIG. 3(e), by RIE (Reactive Ion Etching), thesemiconductor substrate 2 at openings in the resist pattern R1 isremoved to form the through holes A, B, C, D, K, L, H and N and toperform sensor chip separation (cut out individual sensor chips 1 fromthe wafer). Then, by means of an organic solvent, the resist pattern isremoved, the wax is dissolved, and the dummy wafer 21 is detached fromthe semiconductor substrate 2. Finally, washing is carried out and thesensor chip, that is, the six-axis force sensor 1, is completed.

[0124] Although in the process described above an impurity wasintroduced into the semiconductor substrate 2 by ion-implantation,thermal diffusion may alternatively be used to introduce the impurity.Although sputtering was employed as the method for depositing the metalfilm, a vapor deposition method, anticipating lift-off, mayalternatively be used.

[0125] As described above, in the sensor chip 1, because semiconductorresistance devices for detecting strain are formed in the surface of asemiconductor substrate using photolithography processes, the resistancedevices can be disposed exactly in the required positions. Accordingly,devices with the performance specified for them in design can bemanufactured easily with good reproducibility, and productivity is alsoimproved. As a result, compared to a related art multi-axis force sensormanufactured by a process involving an affixing step, size reduction ofa six-axis force sensor 1 according to this preferred embodiment iseasy, and its reproducibility and manufacturability are also excellent.

[0126] Also, because the six-axis force sensor 1 has a constructionwherein resistance devices are formed on a semiconductor substrate 2,the sensor can be made small and thin. The manufacturing method of thesix-axis force sensor 1 is such that the resistance devices can bereduced in size to the resolution limit of photolithography. Thus asix-axis force sensor can be made smaller and thinner and it is possibleto realize a six-axis force sensor with a reduced influence of otheraxis interference.

[0127] Next, the detection/measurement operation of this sensor chip 1constituting a six-axis force sensor will be explained. First, theprinciple of detection/measurement of six components of force and momentresulting from an external force applied to the sensor chip 1 will beexplained using FIG. 4 through FIG. 11.

[0128] Here, with reference to FIG. 4 and FIG. 5, definitions forillustrating the external force detection principle of the sensor chip 1will be explained. FIG. 4 is a simplified perspective view of the sensorchip 1, and FIG. 5 is a simplified plan view of the sensor chip 1. Thesensor chip 1, which is a semiconductor sensor device, is square in planand shaped like a flat plate. The sensor chip 1 is made up of a centralaction part 4, a support part 3 surrounding the action part 4, and fourconnecting parts 5A, 5B, 5C and 5D positioned between the action part 4and the support part 3. Each of the four connecting parts 5A to 5D is aT-shaped beam and has a bridge part and an elastic part. As describedabove, each of the four connecting parts 5A to 5D has disposed on oneface thereof, near its boundary with the action part 4, three strainresistance devices (Sxa1, Sxa2, Sxa3), (Sxb1, Sxb2, Sxb3), (Sya1, Sya2,Sya3) or (Syb1, Syb2, Syb3).

[0129] In FIG. 4 and FIG. 5, with respect to the sensor chip 1,orthogonal X, Y and Z axes are shown. In FIG. 4, upper, lower, left andright sides of the sensor chip 1 are set for convenience. In FIG. 5, thehorizontal axis is shown as the X-axis and the vertical axis is shown asthe Y-axis. Also, in FIG. 4, a force and a moment pertaining to eachaxis are represented with arrows and reference letters. With respect tothe three orthogonal coordinate axes (X-axis, Y-axis, Z-axis), a forcein the X-axis direction is denoted Fx; a force in the Y-axis directionFy; and a force in the Z-axis direction Fz. Similarly, a moment actingabout the X-axis is denoted Mx; a moment acting about the Y-axis My; anda moment acting about the Z-axis Mz.

[0130] Next, with reference to FIG. 6(a) through FIG. 11(b), typicalaxis forces (forces and moments) on the sensor chip 1, and variations,will be discussed specifically. FIG. 6(a) is a plan view of the obverseside of the sensor chip 1 in a deformed state, and FIG. 6(b) is acorresponding cross-sectional view, showing a state of deformation ordisplacement of the action part 4 of the sensor chip 1. The pairs ofFIGS. 7(a), (b) through FIGS. 11(a), (b) are also plan andcross-sectional views similar to those of FIGS. 6(a) and (b).

[0131] In FIGS. 6(a) and (b), a force −Fy is being applied to the actionpart 4 of the sensor chip 1 along the Y-axis, downward from centrallyabove. Consequently, a strain arises in each of the joining portionsbetween the connecting parts 5B, 5D and the action part 4, and in thejoining portions between the connecting parts 5A and 5C and the actionpart 4 the force −Fy is absorbed by the elastic parts of the connectingparts and the occurrence of strain is suppressed. At this time, in eachof the strain resistance devices Sxa3 and Sxb1 a compressive strainarises and the resistance value falls; in Sxa1 and Sxb3 a tensile strainarises and the resistance value rises; and in Sya1-Sya3 and Syb1-Syb3the change in resistance value is extremely small.

[0132] In FIGS. 7(a) and (b), a force Fx is being applied to the actionpart 4 of the sensor chip 1 along the X-axis, rightward from centrallyon the left. Consequently, a strain arises in each of the joiningportions between the connecting parts 5A, 5C and the action part 4, andin the joining portions between the connecting parts 5B and 5D and theaction part 4 the force Fx is absorbed by the elastic parts of theconnecting parts and the occurrence of strain is suppressed. At thistime, in each of the strain resistance devices Sya3 and Syb1 acompressive strain arises and the resistance value falls; in Sya1 andSyb3 a tensile strain arises and the resistance value rises; and inSxa1-Sxa3 and Sxb1-Sxb3 the change in resistance value is extremelysmall.

[0133] In FIGS. 8(a) and (b), a force −Fz is being applied to the actionpart 4 of the sensor chip 1 along the Z-axis (perpendicular to the planeof the paper). Consequently, a strain arises in each of the joiningportions between the connecting parts 5A-5D and the action part 4. Atthis time, in all of the strain resistance devices the same compressivestrain arises and the resistance value falls.

[0134] In FIGS. 9(a) and (b), a moment My is being applied to the actionpart 4 of the sensor chip 1 about the Y-axis in the direction of thearrow. Consequently, a strain arises in each of the joining portionsbetween the connecting parts 5B, 5D and the action part 4, and in thejoining portions between the connecting parts 5A, 5C and the action part4 the moment My is absorbed by the elastic parts of the connecting partsand the occurrence of strain is suppressed. At this time, in each of thestrain resistance devices Sxa1-Sxa3 a compressive strain arises and theresistance value falls; in Sxb1-Sxb3 a tensile strain arises and theresistance value rises; and in Sya1-Sya3 and Syb1-Syb3 the change inresistance value is extremely small.

[0135] In FIGS. 10(a) and (b), a moment Mx is being applied to theaction part 4 of the sensor chip 1 about the X-axis in the direction ofthe arrow. Consequently, a strain arises in each of the joining portionsbetween the connecting parts 5A, 5C and the action part 4, and in thejoining portions between the connecting parts 5B, 5D and the action part4 the moment Mx is absorbed by the elastic parts of the connecting partsand the occurrence of strain is suppressed. At this time, in each of thestrain resistance devices Syb1-Syb3 a compressive strain arises and theresistance value falls; in Sya1-Sya3 a tensile strain arises and theresistance value rises; and in Sxa1-Sxa3 and Sxb1-Sxb3 the change inresistance value is extremely small.

[0136] In FIGS. 11(a) and (b), a moment Mz is being applied to theaction part 4 of the sensor chip 1 about the Z-axis in the direction ofthe arrow (counterclockwise). Consequently, a strain arises in each ofthe joining portions between the connecting parts 5A-5D and the actionpart 4. At this time, in each of the strain resistance devices Sya1,Sxb1, Syb1, Sxa1 a compressive strain arises and the resistance valuefalls, and in Sya3, Sxb3, Syb3, Sxa3 a tensile strain arises and theresistance value rises.

[0137] Next, a prototype example of the six-axis force sensor 1 will nowbe described. Parameters of the constituent parts of a six-axis forcesensor 1 actually made will be shown.

[0138] A semiconductor substrate 2 forming the six-axis force sensor(sensor chip 1) was a P(phosphorus)-doped n-type silicon substrate, andits resistivity was 1 Ωcm. The semiconductor substrate 2 was an n-type(100) silicon substrate, and p-type active layers S constituting theresistance devices were formed with their long axes directions parallelwith either the <011> direction or the <0-11> direction. The sheetresistance (ρ) of the active layers S, which were formed byion-implantation, was 500 [Ω/□], and the resistance devices Sxa1-Sxa3,Sya1-Sya3, Sxb1-Sxb3, Syb1-Syb3 of this trial six-axis force sensor 1were formed with a resistance value of 53 kΩ.

[0139] FIGS. 12(a) through (c) show the shapes and dimensions of theconstituent parts of this prototype example of the six-axis force sensor1. The dimensions shown in FIGS. 12(a) through (c) are all inmillimeters. FIG. 12(a) is a plan view. The chip dimensions of thesemiconductor substrate 2, i.e. the six-axis force sensor 1, are 10mm×10 mm, and the size of the action part 4 is 3 mm×3 mm. Bridge parts5Aa, 5Ba, 5Ca and 5Da of width 0.4 mm and length 1.325 mm are formedextending from the centers of the sides of the action part 4 to thesupport parts 3 opposite.

[0140] The holes A, B, C and D are formed with dimensions of width 0.2mm, length 5.25 mm. The elastic parts 5Ab, 5Bb, 5Cb and 5Db are alsoformed with dimensions of width 0.2 mm and length 5.25 mm, and each hasone end of the respective bridge part 5Aa, 5Ba, 5Ca or 5Da connected toit at its center.

[0141]FIG. 12(b) shows the positional relationships in which theresistance devices (Sxa1-Sxa3, Sya1-Sya3, Sxb1-Sxb3 and Syb1-Syb3) areformed. In the figure, the resistance devices Sya1-Sya3 formed at thejoining portion between the bridge part 5Aa and the action part 4 areshown. The resistance devices Sya1-Sya3 are disposed near the joiningportion between the bridge part 5Aa and the action part 4 with a widthof 0.07 mm and at a pitch of 0.07 mm.

[0142] As shown in FIG. 12(c), each of the resistance devices consistsof four 0.3 mm lengths of an active layer S of width 0.01 mm, formed ata pitch of 0.01 mm in an area of width 0.07 mm.

[0143] Returning to FIG. 12(b), the resistance devices Sya1-Sya3 aredisposed at the joining portion between the bridge part 5Aa and theaction part 4 with the long axis directions of their active layers Sparallel with the long axis direction of the bridge part 5Aa. Theseresistance devices Sya1-Sya3 are disposed with their long axis directioncenters in a position level with the side of the action part 4 (they areformed extending 0.15 mm above and below the side of the action part 4).The resistance device Sya2 is disposed in the center of the short axisdirection of the bridge part 5Aa, and the resistance devices Sya1 andSya3 are formed with their outer sides, which do not face the sides ofthe Sya2, in positions 0.025 mm from the short axis direction ends (inother words the sides) of the bridge part 5Aa.

[0144] The resistance devices Sxa1-Sxa3, Sxb1-Sxb3 and Syb1-Syb3 formedat the joining portions between the bridge parts 5Ba, 5Ca and 5Da andthe action part 4 are also formed with similar disposition anddimensions to the resistance devices Sya1-Sya3 discussed above.

[0145] Next, the sensor characteristics with which force and momentcomponents applied to the six-axis force sensor 1 are detected will bediscussed.

[0146] When the six axis components (axis forces) mentioned above, i.e.Fx[N], Fy[N], Fz[N], Mx[N.cm], My[N.cm], Mz[N.cm], are applied to thesix-axis force sensor 1 on its own, the relationship between these sixaxis components and the detection signals from the six-axis force sensor1 is as follows.

[0147] An actual six-axis force sensor is made up of a sensor chip 1 andan external measuring device for computing signals pertaining to theresistance change proportions obtained from the twelve resistancedevices of the sensor chip 1, and is constructed as a six-axis forcesensor apparatus. The signals (computed resistance change proportions)finally outputted from the six-axis force sensor via the computation ofthe external measuring device are the six signals Sig1, Sig2, Sig3,Sig4, Sig5 and Sig6. If the values of the resistance change proportionsobtained from the twelve resistance devices Sxa1-Sxa3, Sya1-Sya3,Sxb1-Sxb3 and Syb1-Syb3 of the sensor chip 1 are expressed R′Sxa1,R′Sxa2, R′Sxa3, R′Sya1, R′Sya2, R′Sya3, R′Sxb1, R′Sxb2, R′Sxb3, R′Syb1,R′Syb2, R′Syb3, then the above-mentioned six signals Sig1-Sig6 can bedetermined on the basis of the following formulas.

Sig1=((R′Sya1−R′Sya3)+(R′Syb3−R′Syb1))/4  (1)

Sig2=((R′Sxa3−R′Sxa1)+(R′Sxb1−R′Sxb3))/4  (2)

Sig3=(R′Sxa2+R′Sya2+R′Sxb2+R′Syb2)/4  (3)

Sig4=(R′Sya2−R′Syb2)/2  (4)

Sig5=(R′Sxb2−R′Sxa2)/2  (5)

Sig6=((R′Sxa3−R′Sxa1)+(R′Sya3−R′Sya1)+(R′Sxb3−R′Sxb1)+(R′Syb3−R′Syb1))/8  (6)

[0148] The six output signals Sig1-Sig6 of the six-axis force sensordetermined according to the above formulas (1)-(6) on the basis of theresistance change proportions of the twelve resistance devices and thesix axis components Fx, Fy, Fz, Mx, My, Mz applied to the sensor chip 1,when the relationships between the two are obtained experimentally byfinding the output signals of the six-axis force sensor corresponding tospecific axis force components, can be related by the matrix table shownin FIG. 13. FIG. 13 shows relationships between the forces Fx, Fy, Fzand the moments Mx, My, Mz applied to the action part 4 and the computedresistance change proportions Sig1-Sig6 per unit (of force of moment).In FIG. 13, the area 100 on the left side shows the above-mentionedsignals Sig1-Sig6 and the formulas (1)-(6). FIG. 13 was obtained byexperimental measurements.

[0149] The above-mentioned formulas (1)-(6) are made up of resistancechange proportions of resistance devices extended (subjected to tension)and resistance devices compressed by strains arising in the joiningportions between the bridge parts and the action part by an applicationof an external force.

[0150] As is clear from the matrix of FIG. 13, the resistance devicesused in the formulas (1)-(6) are selected so that the non-diagonalcomponents of the matrix either are “0” or are small numbers comparedwith the diagonal components 200. That is, the formulas (1)-(6) arecomposed with selected resistance devices to so perform the computationof the computed resistance change proportions that when the force ormoment of a certain axis is measured, to prevent other axisinterference, resistance change proportions pertaining to the forces andmoments of the other axes cancel each other out.

[0151] Looking at formula (1), which corresponds to the application of aforce Fx, when Fx is applied, because a tensile stress is applied toeach of the resistance devices Sya1 and Syb3 and a compressive stress isapplied to each of the resistance devices Sya3 and Syb1, as shown by thematrix element with the reference number 201 in FIG. 13 the computedresistance change proportion is 0.00431(1/N) per unit Fx.

[0152] Still looking at this formula (1) corresponding to theapplication of a force Fx, when a force Fy is applied, a tensile stressis applied to both the resistance device Sya1 and the resistance deviceSya3 and a compressive stress is applied to both the resistance deviceSyb3 and the resistance device Syb1, and consequently the resistancechange proportions R′Sya1 and R′Sya3 and the resistance changeproportions R′Syb3 and R′Syb1 cancel each other out. As a result, whenFy is applied, the computed resistance change proportion Sig1 istheoretically “0” in formula (1).

[0153] When in formula (1) a force Fz is applied, a tensile stress actson all of the resistance devices Sya1, Sya3, Syb3 and Syb1, and againthe computed resistance change proportion Sig1 given by formula (1) istheoretically “0”.

[0154] When in formula (1) a moment Mx is applied, a tensile stress isapplied to both the resistance device Sya1 and the resistance deviceSya3 and a compressive stress is applied to both the resistance deviceSyb3 and the resistance device Syb1, and again the computed resistancechange proportion Sig1 in formula (1) is theoretically obtained as “0”.

[0155] When in formula (1) a moment My is applied, because a compressivestress acts on the resistance devices Sya1 and Sya3 and a tensile stressacts on Syb3 and Syb1, the computed resistance change proportion Sig1 isnot theoretically “0”. However, because the elastic parts 5Ab and 5Cbabsorb the strain to some extent, it is a small number compared to thevalue of the computed resistance change proportion of when there is aninput Fx.

[0156] When in formula (1) a moment Mz is applied, a tensile stress isapplied to both the resistance device Sya1 and the resistance deviceSyb1 and a compressive stress is applied to both the resistance deviceSya3 and the resistance device Syb3, the resistance change proportionsof these resistance devices cancel each other out as (R′Sya1-R′Sya3) and(R′Syb3-R′Syb1), and the computed resistance change proportion Sig1 informula (1) is again theoretically obtained as “0”.

[0157] When from these results an inverse matrix is obtained and therelationship between Fx and the computed resistance change proportionsis found, as explained above, this relationship becomes substantiallyFx=Sig1×[diagonal element 11] (corresponding to m′11 in the related artexamples).

[0158] Because resistance devices are selected so that the values of thenon-diagonal elements become “0” or very small compared with thediagonal elements and resistance change proportions of these resistancedevices are used in the formula for obtaining the computed resistancechange proportion Sig1 like this, the probability of other axisinterference arising can be greatly reduced. And in the other formulas(2)-(6) also, the resistance devices used for measurement are similarlychosen so that the non-diagonal elements become “0” or become small withrespect to the diagonal elements.

[0159] The kind of resistance change proportion canceling describedabove is possible because of the way that, on each of the bridge parts,twelve resistance devices are disposed in symmetrical positions oneither side of a resistance device disposed centrally.

[0160] Here, in each of the formulas, the numerator is divided by thenumber of resistances used in the formula in order to standardize theresistance change proportions to a proportion of one resistance.

[0161] Results of obtaining computed resistance change proportionsrespective to the application of each of the force and moment componentsare shown in the table of FIG. 13. In the above-mentioned formulas(1)-(6), for the computed resistance change proportions Sig1, Sig2 andSig3 computed resistance change proportions per IN are obtained, and forthe computed resistance change proportions Sig4, Sig5 and Sig6 computedresistance change proportions per 1 N.cm are obtained.

[0162] Here, the method by which the resistance change proportion of aresistance device on its own is obtained will be described. If forexample the resistance value of the resistance device Sxa1 (under astress) is written RSxa1, its resistance change proportion is writtenR′Sxa1, and the resistance value of the resistance device 13, which isthe compensation resistance, is written Rcomp, then the true resistancevalue of the resistance device Sxa1 is ‘RSxa1.Rcomp(0)/Rcomp’. Here,Rcomp(0) is the resistance value of the resistance device 13 at roomtemperature, and Rcomp is its resistance value at the ambienttemperature.

[0163] The resistance change proportion R′Sxa1 is obtained by thefollowing formula.

R′Sxa1=((RSxa1.Rcomp(0)/Rcomp)−Sxa1(0))/Sxa1(0)

[0164] Here, the resistance value Sxa1(0) is the resistance value of theresistance device Sxa1 when no stress is acting on the resistance deviceSxa1.

[0165] Also for the other resistance devices Sxa2, Sxa3, Sxb1-Sxb3,Sya1-Sya3 and Syb1-Syb3, resistance change proportions R′Sxa2, R′Sxa3,R′Sxb1-R′Sxb3, R′Sya1-R′Sya3, R′Syb1-R′Syb3 can be obtained in the sameway as the resistance change proportion R′Sxa1 of the resistance deviceSxa1 above. The resistance values are obtained from current-voltagecharacteristics measured at the signal electrodes 10.

[0166] On the basis of the matrix table of FIG. 13 obtained byexperimental measurement, an equation (7) for obtaining the forces andmoments of an applied external force from the computed resistance changeproportions Sig1-Sig6 measured by the six-axis force sensor 1 can bewritten. $\begin{matrix}{\begin{pmatrix}{+ {{Fx}\lbrack N\rbrack}} \\{+ {{Fy}\lbrack N\rbrack}} \\{+ {{Fz}\lbrack N\rbrack}} \\{{+ M}\quad {x\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{My}\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{Mz}\left\lbrack {N \cdot {cm}} \right\rbrack}}\end{pmatrix} = {\begin{pmatrix}231.7 & 0 & 0 & 0 & 6.3 & 0 \\0 & 231.7 & 0 & {- 6.3} & 0 & 0 \\0.1 & 0.2 & 146.4 & 1.1 & 1.1 & {- 1.7} \\0 & 1.0 & 0 & 23.6 & 0 & 0 \\{- 1.0} & 0 & 0 & 0 & 23.6 & 0 \\0 & 0 & 0 & 0 & 0 & 84.2\end{pmatrix} \times \begin{pmatrix}{Sig1} \\{Sig2} \\{Sig3} \\{Sig4} \\{Sig5} \\{Sig6}\end{pmatrix}}} & (7)\end{matrix}$

[0167] The matrix that is the left term on the right side of equation(7) is obtained as the inverse matrix of the elements in the table ofFIG. 13. That is, this inverse matrix is the matrix for obtaining therelationships between the computed resistance change proportions and theforces and moments axis by axis, and by applying the computed resistancechange proportions Sig1-Sig6 to equation (7) it is possible to obtainforce and moment components.

[0168] Because the non-diagonal elements in the matrix on the right sideof equation (7) either are small values compared to the values of thediagonal elements or are “0”, other axis interference among the axes issuppressed. That is, it is possible to construct a matrix showing a(computed resistance change proportion−force and moment) relationshipwhich suppresses other axis interference so that for example when aforce Fx is applied only the value of the force Fx is obtained as thecomputation result of formula (1).

[0169] Thus, compared to related art, in which there has tended to be aproblem of reproducibility of measurement values due to other axisinterference, with a six-axis force sensor 1 according to this inventionit is possible to obtain a matrix showing a (computed resistance changeproportion−force and moment) relationship wherein the non-diagonalelements either are “0” or take very small values compared with thediagonal elements, and by suppressing other axis interference in thisway it is possible to improve robustness and reproducibility ofmeasurement results.

[0170] Next, measurement in a case where resistance devices are formednot only on the obverse side of the semiconductor substrate 2, asdescribed above, but also on its reverse side will be described, usingthe example of a six-axis force sensor made for trials.

[0171]FIG. 14 shows the positions of resistance devices fabricated onthe reverse side of a semiconductor substrate 2 forming a six-axis forcesensor. This FIG. 14 is a view of the underside of the semiconductorsubstrate 2 as seen through its upper side (the side already discussedwith reference to FIG. 5), and for example a resistance device Sya1u isformed at the back side of the resistance device Sya1 and a resistancedevice Syb1u is formed at the back side of the resistance device Syb1.These resistance devices on the reverse side are manufactured by thesame method as the resistance devices on the front side alreadydiscussed. That is, after the resistance devices on the front side arefabricated, the front side is protected by being covered with somesuitable material, and semiconductor devices are formed on the rear sideby similar processes as those carried out on the front side, after whichthe protective material is removed from the front side.

[0172] The characteristics of a six-axis force sensor 1 made usingtwelve resistance devices on the front side and four resistance deviceson the rear side will now be discussed.

[0173] In the same way as that described above with reference to FIG.13, the diagonal elements 210 of the table shown in FIG. 15 are obtainedon the basis of the following formulas (8)-(13). The formulas (8)-(13)are also written in the area 110 of FIG. 15. In the making of the tableof FIG. 15, all of the resistance devices on the front side and aresistance device Sya2u on the connecting part 5A, a resistance deviceSxa2u on the connecting part 55, a resistance device Syb2u on theconnecting part 5C and a resistance device Sxb2u on the connecting part5D on the rear side were used. Although twelve resistance devices areshown in FIG. 14, in the making of the table shown in FIG. 15, asix-axis force sensor 1 construction having on its rear side only theresistance devices Sya2u, Sxa2u, Syb2u and Sxb2u, disposed in thepositions shown in FIG. 14, was assumed.

Sig1=((R′Sxb2+R′Sxb2u)−(R′Sxa2+R′Sxa2u))/4  (8)

Sig2=((R′Syb2+R′Syb2u)−(R′Sya2−R′Sya2y))/4  (9)

Sig3=((R′Sxa2−R′Sxa2u)+(R′Sya2−R′Sya2u)+(R′Sxb2−R′Sxb2u)+(R′Syb2−R′Syb2u))/8  (10)

Sig4=((R′Sya2−R′Sya2u)−(R′Syb2−R′Syb2u))/4  (11)

Sig5=((R′Sxb2−R′Sxb2u)−(R′Sxa2−R′Sxa2u))/4  (12)

Sig6=((R′Sxa3−R′Sxa1)+(R′Sya3−R′Sya1)+(R′Sxb3−R′Sxb1)+(R′Syb3−R′Syb1))/8  (13)

[0174] In obtaining the computed resistance change proportions Sig1-Sig6of the respective force and moment components, as in the formulas(1)-(6), in the formulas (8)-(13) also, combining of resistance devicessuch that other axis interference does not occur is carried out.

[0175] Whereas with resistance devices provided only on the front sideof the six-axis force sensor 1 it was not possible to make all of thenon-diagonal elements “0”, as shown in the table of FIG. 13, by alsousing resistance devices disposed on the rear side of the substrate, itbecomes possible to group the resistance devices in the formulas(8)-(13) so that all of the non-diagonal elements become “0”, as shownin the table of FIG. 15.

[0176] For example, when a force Fx is applied to the action part 4, informula (8), which is for obtaining the computed resistance changeproportion Sig1, a computed resistance change proportion as pertainingto one resistance device is obtained by combinations of the resistancedevices Sxb2 and Sxa2 and the resistance devices Sxb2u and Sxa2u, whichface each other on the front and rear sides of the six-axis force sensor1. When the force Fx acts on the action part 4, a tensile force, albeitsmall, acts on each of the resistance devices Sxb2 and Sxb2u, and acompressive force, albeit small, acts on the resistance devices Sxa2,Sxa2u.

[0177] For example, when a moment My is applied to the action part 4, inthe case where there are resistance devices on the front side of thesubstrate only, a number, albeit a very small value, appears as otheraxis interference in the computed resistance change proportion Sig1 fordetecting a force Fx as shown in FIG. 13. However, it resistance devicesare provided on the rear side of the substrate and suitable resistancedevices among these are selected and used in formula (8) based on theresistance change proportion of the resistance element to compute thecomputed resistance change proportion Sig1, when a moment My is appliedthe resistance change proportions R′Sxb2 and R′Sxb2u cancel each otherout and the resistance change proportions R′Sxa2 and R′Sxa2u cancel eachother out. Consequently, the value of the computed resistance changeproportion Sig1 constituting the non-diagonal element with respect tothe moment My is “0”

[0178] In the same way as the equation (7) was obtained, by finding theinverse matrix of the matrix in the table of FIG. 15, a matrix showing a(computed resistance change proportion−force and moment) relationshipcan be obtained.

[0179] On the basis of this (computed resistance change proportion−forceand moment) relationship matrix, it is possible to write the following(computed resistance change proportion−force and moment) relationshipequation (14). $\begin{matrix}{\begin{pmatrix}{+ {{Fx}\lbrack N\rbrack}} \\{+ {{Fy}\lbrack N\rbrack}} \\{+ {{Fz}\lbrack N\rbrack}} \\{{+ M}\quad {x\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{My}\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{Mz}\left\lbrack {N \cdot {cm}} \right\rbrack}}\end{pmatrix} = {\begin{pmatrix}5263.2 & 0 & 0 & 0 & 0 & 0 \\0 & 5263.2 & 0 & 0 & 0 & 0 \\0 & 0 & 146.8 & 0 & 0 & 0 \\0 & 0 & 0 & 23.6 & 0 & 0 \\0 & 0 & 0 & 0 & 23.6 & 0 \\0 & 0 & 0 & 0 & 0 & 84.2\end{pmatrix} \times \begin{pmatrix}{Sig1} \\{Sig2} \\{Sig3} \\{Sig4} \\{Sig5} \\{Sig6}\end{pmatrix}}} & (14)\end{matrix}$

[0180] From the above, in equation (14), by multiplying a column vectorof computed resistance change proportions obtained with theabove-mentioned formulas (8)-(13) using measured resistance changeproportions by the above-mentioned matrix pertaining to the (computedresistance change proportion−force and moment) relationship, it ispossible to obtain an applied external force separated into sixcomponents of force and moment.

[0181] As is clear from the (computed resistance change proportion−forceand moment) relationship matrix on the right side of equation (14), thenon-diagonal elements are all “0”, calculation is easy, other axisinterference can be greatly suppressed, robustness and reproducibilityof measured values improves, and measurement sensitivity and measurementprecision increase.

[0182] Compared to the matrix in equation (7), the sizes of the diagonalelements in equation (14) are different. However, this is because theseare values obtained by experimental measurement; in a six-axis forcesensor according to the invention these values can be suitably adjustedby adjustment of the thickness of the substrate, the width of the bridgeparts, and the disposition of the resistances, and the measurementsensitivity can be adjusted as necessary according to the applicationand also by axis direction.

[0183] Next, characteristics of a six-axis force sensor 1 constructedusing twelve resistance devices on the front side and twelve resistancedevices on the rear side will be discussed. By the same method as thatexplained in connection with the table shown in FIG. 13, diagonalelements 220 of the table shown in FIG. 16 are obtained on the basis ofthe following formulas (15)-(20). These formulas are also shown in area120 of FIG. 16. In the making of the table shown in FIG. 16, all of theresistance devices on the front side of the six-axis force sensor andall of the resistance devices on the rear side shown in FIG. 14, namelythe resistance devices Sya1u-Sya3u on the conecting part 5A, theresistance devices Sxa1u-Sxa3u on the connecting part 5B, the resistancedevices Syb1u-Syb3u on the connecting part 5C and the resistance devicesSxb1u-Sxb3u on the connecting part 5D, are used.

Sig1=((R′Sya1−R′Sya3)+(R′Syb3−R′Syb1)+(R′Sya1u−R′Sya3u)+(R′Syb3u−R′Syb1u))/8  (15)

Sig2=((R′Sxa3−R′Sxa1)+(R′Sxb1−R′Sxb3)+(R′Sxa3u−R′Sxa1u)+(R′Sxb1u−R′Sxb3u))/8  (16)

Sig3=((R′Sxa2−R′Sxa2u)+(R′Sya2−R′Sya2u)+(R′Sxb2−R′Sxb2u)+(R′Syb2−R′Syb2u))/8  (17)

Sig4=((R′Sya2−R′Sya2u)−(R′Syb2−R′Syb2u))/4  (18)

Sig5=((R′Sxb2−R′Sxb2u)−(R′Sxa2−R′Sxa2u))/4  (19) $\begin{matrix}\begin{matrix}{{Sig6} = \left( {\left( {{R^{\prime}{Sxa3}} - {R^{\prime}{Sxa1}}} \right) + \left( {{R^{\prime}{Sya3}} - {R^{\prime}{Sya1}}} \right) +} \right.} \\{{\left( {{R^{\prime}{Sxb3}} - {R^{\prime}{Sxb1}}} \right) + \left( {{R^{\prime}{Syb3}} - {R^{\prime}{Syb1}}} \right) +}} \\{{\left( {{R^{\prime}{Sxa3u}} - {R^{\prime}{Sxa1u}}} \right) + \left( {{R^{\prime}{Sya3u}} - {R^{\prime}{Sya1u}}} \right) +}} \\{\left. {\left( {{R^{\prime}{Sxb3u}} - {R^{\prime}{Sxb1u}}} \right) + \left( {{R^{\prime}{Syb3u}} - {R^{\prime}{Syb1u}}} \right)} \right)/16}\end{matrix} & (20)\end{matrix}$

[0184] Here, in obtaining the computed resistance change proportionsSig1-Sig6 of the respective force and moment components, as with theformulas (1)-(6) and the formulas (8)-(13), in the above formulas(15)-(20), resistance devices are used in such combinations that otheraxis interference does not occur.

[0185] Whereas when resistance devices were only provided on the frontside of the six-axis force sensor 1 it was not possible to bring all ofthe non-diagonal elements to “0”, as shown in the table of FIG. 13,here, as shown in FIG. 16, as in the case of FIG. 15, it is possible tocombine the resistance devices in the formulas (15)-(20) so that all ofthe non-diagonal elements become “0”. Also, because the valuescorresponding to the resistance devices Sya1 and Sya2 among the diagonalelements 220 can be made larger than in the table of FIG. 15, thesensitivity with which the forces Fx and Fy are detected can beincreased.

[0186] For example, when a force Fx is applied to the action part 4, inthe formula (15) for obtaining Sig1, a computed resistance changeproportion as pertaining to one resistance device is obtained by usingcombinations of the resistance devices Sya1, Sya3, Syb1 and Syb3 and theresistance devices Sya1u, Sya3u, Syb1u and Syb3u, which face each otheron the front and rear sides of the substrate. When a force Fx is appliedto the action part 4, a tensile force acts on each of the resistancedevices Sya1, Syb3, Sya1u and Syb3u and a compressive force acts on eachof the resistance devices Sya3, Syb1, Sya3u and Syb1u. At this time,because as shown in formulas (8) and (9) the resistance devices wherethe force Fx is absorbed by an elastic part are not used, the appliedexternal force can be separated into force and moment components withoutthe measurement sensitivity falling, and the precision and sensitivitywith which the applied external force is measured can be increased. Inthe same way as equation (7) was obtained, a (computed resistance changeproportion−force and moment) relationship matrix is obtained by findingthe inverse matrix of the matrix in the table of FIG. 16. On the basisof this (computed resistance change proportion−force and moment)relationship matrix the following equation (21), which is a (computedresistance change proportion−force and moment) relationship equation, iswritten. $\begin{matrix}{\begin{pmatrix}{+ {{Fx}\lbrack N\rbrack}} \\{+ {{Fy}\lbrack N\rbrack}} \\{+ {{Fz}\lbrack N\rbrack}} \\{{+ M}\quad {x\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{My}\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{+ {{Mz}\left\lbrack {N \cdot {cm}} \right\rbrack}}\end{pmatrix} = {\begin{pmatrix}232.0 & 0 & 0 & 0 & 0 & 0 \\0 & 232.0 & 0 & 0 & 0 & 0 \\0 & 0 & 146.8 & 0 & 0 & 0 \\0 & 0 & 0 & 23.6 & 0 & 0 \\0 & 0 & 0 & 0 & 23.6 & 0 \\0 & 0 & 0 & 0 & 0 & 84.2\end{pmatrix} \times \begin{pmatrix}{Sig1} \\{Sig2} \\{Sig3} \\{Sig4} \\{Sig5} \\{Sig6}\end{pmatrix}}} & (21)\end{matrix}$

[0187] From the above, in equation (21), by multiplying a column vectorof computed resistance change proportions measured by experiment by the(computed resistance change proportion−force and moment) relationshipmatrix, it is possible to obtain an applied external force separatedinto six components of force and moment.

[0188] As is clear from the (computed resistance change proportion−forceand moment) relationship matrix on the right side of equation (21), thenon-diagonal elements are all “0” and other axis interference is greatlysuppressed; consequently, robustness and reproducibility of measuredvalues improve, and because also the values of the diagonal elements aremore similar to each other than those in equation (14), measurementsensitivity and measurement precision are further increased.

[0189] Also, in a six-axis force sensor according to this preferredembodiment, these figures can be freely adjusted by adjustment of thethickness of the substrate, the width of the bridge part and thedisposition of the resistance devices, and the measurement sensitivitycan be adjusted as necessary according to the application and also byaxis direction.

[0190] The parameters and shape of the six-axis force sensor and thedimensions of its constituent parts are to be optimized as necessaryaccording to the application in which the sensor is to be used, and thevarious figures given above are no more than one example. Although aspecific preferred embodiment of the invention has been described abovein some detail with reference to drawings, the specific construction ofa sensor according to the invention is not limited to this preferredembodiment, and various design changes can be made within the scope ofthe invention.

[0191] In the foregoing first preferred embodiment the dispositions ofthe semiconductor resistances only constitute one example, and similareffects can be obtained by any disposition of resistance devices withwhich it is possible to prevent other axis interference by obtainingcombinations of resistance devices such that non-diagonal elements in amatrix showing a computed resistance change proportion−force and momentrelationship cancel each other out and to obtain formulas for computingcomputed resistance change proportions corresponding to forces andmoments like the formulas (1)-(6), (8)-(13) and (15)-(20).

[0192] In this first preferred embodiment, a structure wherein then-type semiconductor substrate 2 of the six-axis force sensor 1 is ofcrystal direction (100)-plane and the electrical current axes ofsemiconductor resistance devices consisting of p-type active layers Sare disposed in the <011> or <0-11> direction was described. However,alternatively a semiconductor substrate 2 of crystal direction(110)-plane can be used and the electrical current axes of thesemiconductor resistance devices consisting of p-type active layers Sare disposed in the <1-10> or <00-1> direction. Or a semiconductorsubstrate 2 of crystal direction (110)-plane can be used and theelectrical current axes of the semiconductor resistance devicesconsisting of p-type active layers S are disposed in the <1-1-1> or<-11-2> direction.

[0193] By means of these combinations of crystal direction and currentaxes it is possible to obtain larger resistance changes with respect tothe same strains, and it is possible to improve the precision with whichthe force and moment components of an applied external force can bemeasured.

[0194] As a second preferred embodiment of the invention, it is possibleto obtain the same results as those of the preferred embodimentdescribed above in a six-axis force sensor having the structure shown inFIG. 17. FIG. 17 is a plan view showing the construction of a six-axisforce sensor 300 constituting a second preferred embodiment. Thissix-axis force sensor 300 is made up of hole regions 301-304, supportparts 305, an action part 306, elastic parts 307, 308, 309, 310 andbridge parts 311, 312, 313 and 314.

[0195] The layout and number of resistance devices is the same as in thefirst preferred embodiment described above, and the method by which thecomputed resistance change proportions and force and moment componentsare obtained is also the same as in the first preferred embodiment.

[0196] The point in which this second preferred embodiment differs fromthe first preferred embodiment is that there is no equivalent of thehole regions A, B, C and D, and the elastic parts 307, 308, 309 and 310are given their low rigidity by being made thin. When the supportingparts 305 are fixed to a mount, the elastic parts 307, 308, 309 and 310operate as elastic bodies in the same way as the elastic parts 5Ab, 5Bb,5Cb and 5Db of the preferred embodiment described above.

[0197] A third preferred embodiment of a six-axis force sensor accordingto the invention will now be described, with reference to FIG. 18through FIG. 26.

[0198]FIG. 18 is a side view showing schematically a six-axis forcesensor according to a third preferred embodiment. A sensor chip 411 hasthe same structure and function as the six-axis force sensor or sensorchip 1 of the first preferred embodiment having twelve resistancedevices on its front side. Like the sensor chip 1, the sensor chip 411has a central action part for receiving an external force, a supportpart supporting this action part, and connecting parts connecting theaction part and the support part together. The sensor chip 411 ismounted on a plinth part 412 by way of insulating members 413. Thesupport part of the sensor chip 411 is fixed to the plinth part 412. Anexternal force application part 415 is mounted on the plinth part 412 byway of a number of buffering pillars 414. The external force applicationpart 415 is a plate-shaped member and is disposed above the sensor chip411. The buttering pillars 414 have the function of a shock-absorbingstructure. The external force application part 415 is connected to theaction part of the sensor chip 411 by the connecting rod 417 through aninsulating member 416.

[0199] Because the front side of the sensor chip 411, on which thedevices are formed, is covered with an insulating protective film, theinsulating member 416 can alternatively be dispensed with.

[0200] With respect to the sensor chip 411, which has the six-axis forcesensor function, the structure made up of the plinth part 412 and thebuffering pillars 414 and the external force application part 415 formsa box frame. This box frame, on the basis of its buffering mechanism,has the function of attenuating the external force before applying it tothe action part of the sensor chip 411.

[0201] In the construction described above, the plinth part 412 and theexternal force application part 415 are made of members having highstrength. The unknown external force (load) F is applied to the externalforce application part 415. The position and attitude of the externalforce application part 415 subject to the external force F change. Asthey occur, these changes in the position and attitude of the externalforce application part 415 are subject to limitation by the bufferingaction of the buffering pillars 414 arranged between the plinth part 412and the external force applicant part 415. Changes in position andattitude arising in the external force application part 415 aretransmitted to the action part of the sensor chip 411 via the connectingrod 417. A force (or moment) limited by the buffering pillars 414 actson the action part of the sensor chip 411. When an external force isinputted to the external force application part 415, a force attenuatedby the buffering mechanism based on the buttering pillars 414 is appliedto the action part of the sensor chip 411. Consequently, even in thecase of an external force which would break the sensor chip 411, whichwas fabricated as a semiconductor sensor device, if inputted directly tothe sensor chip 411, breaking of the sensor chip 411 can be prevented bymeans of the buffering mechanism based on the above-mentioned box frame.On the other hand, whereas with a sensor chip on its own the level offorce which can be detected is limited to a small range from a materialspoint of view, by providing this box frame having a buffering mechanismit is possible to make the detectable range of force large.

[0202] The strain attenuating rate of the distorting body part in thesensor chip 411 having the six-axis force sensor function is differentfor each of the six axis components, and also depends on the structureof the buffering mechanism of the box frame. Therefore, for example whenwanting to adjust the sensitivity of the sensor chip 411, which is thesix-axis force sensor, axis by axis, by optimizing the structure of thebox frame to match the adjustment objectives it is possible to adjustthe six-axis force sensor made up of the sensor chip 411 and the boxframe having the buffering mechanism to required characteristics.

[0203] The above-mentioned insulating members 413, 416 are provided toprevent noise affecting the signals outputted from the sensor chip 411.

[0204] For the sensor chip 411, the signals Sig1-Sig6 determined usingthe above-mentioned formulas (1)-(6) on the basis of resistance changeproportions of the twelve resistance devices formed on the front side ofthe chip and the six axis forces Fx, Fy, Fz, Mx, My, Mz applied to thesensor chip 411, when the relationships between the two are obtainedexperimentally by finding the output signals of the six-axis forcesensor corresponding to specific axis force components, can be relatedby the matrix table shown in FIG. 19.

[0205] When inverse matrix calculation is carried out on the basis ofthe matrix shown in FIG. 19 and six axis components are computed, thefollowing equation (22) is obtained. $\begin{matrix}{\begin{pmatrix}{{Fx}\lbrack N\rbrack} \\{{Fy}\lbrack N\rbrack} \\{{Fz}\lbrack N\rbrack} \\{M\quad {x\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{{My}\left\lbrack {N \cdot {cm}} \right\rbrack} \\{{Mz}\left\lbrack {N \cdot {cm}} \right\rbrack}\end{pmatrix} = {\begin{pmatrix}1462 & 0 & {.0} & 0 & {- 470} & 0 \\0 & 1462 & 0 & 470 & 0 & 0 \\{- 3} & {- 2} & 833 & 2 & 4 & 115 \\0 & 173 & 0 & 1515 & 0 & 0 \\{- 173} & 0 & 0 & 0 & 1515 & 0 \\0 & 0 & 0 & 0 & 0 & 7692\end{pmatrix} \times \begin{pmatrix}{Sig1} \\{Sig2} \\{Sig3} \\{Sig4} \\{Sig5} \\{Sig6}\end{pmatrix}}} & (22)\end{matrix}$

[0206] As shown by this equation, when the relationship between the sixsignals from the sensor chip 411 and the detected six axis components isviewed as the elements of a matrix, the diagonal elements are dominantand therefore the possibility of problems of other axis interferencearising has been suppressed.

[0207] The sensor chip 411 according to the invention described in thefirst preferred embodiment has reduced other axis interference, isresistant to noise, and has excellent reproducibility in its detectioncharacteristics. On the other hand, when the strains produced by theexternal force applied to the action part become large, the six signalsSig1-Sig6 also become large in proportion with these strains, and ifthis reaches a certain limit value imposed by the nature of the sensorchip as a semiconductor device, the device will break. Therefore, whenthe external force is applied directly to the action part of the sensorchip 411, the sensor chip 411 will break under a relatively smallexternal force. It is to avoid this that in this third preferredembodiment a sensor chip 411 having the detection characteristicsdescribed above is provided with a box frame having a bufferingstructure of the kind described above and shown in FIG. 18. A six-axisforce sensor made up of a sensor chip 411 and a box frame like this hasincreased strength and an enlarged detectable range of external forces.

[0208] Next, a first specific example of a six-axis force sensoraccording to this third preferred embodiment will be described, withreference to FIG. 20 through FIG. 22. This first specific example willmake clear the actual construction of a sensor according to this thirdpreferred embodiment. In FIG. 20 through FIG. 22, elements essentiallythe same as elements described above with reference to FIG. 18 have beengiven the same reference numbers.

[0209] A sensor chip 411 is fixed to a plinth part 412 by its supportpart 422. The sensor chip 411 is disposed at a required height in acentral position on the plinth part 412. The plinth part 412 has foursupport rods 431 erected at its corners, and a square frame plate member432 is mounted on the four support rods 431. The support rods 431 arefixed to the four corners of the square frame plate member 432. Theframe plate member 432 is made of for example aluminum plate. Middlesupport rods 433 are erected at middle positions on the four sides ofthe square frame plate member 432. The four middle support rods 433support an external force application plate 434. The external forceapplication plate 434 is for example an aluminum plate. The center ofthe external force application plate 434 is connected to an action part421 of the sensor chip 411 by a connecting rod 417. An external forceacting on the external force application plate 434 is indirectlytransmitted to the action part 421 of the sensor chip 411 by theconnecting rod 417. The external force application plate 434 issupported by a buffering mechanism made up of the four support rods 431,the frame plate member 432 and the four middle support rods 433. Thisbuffering mechanism attenuates the external force transmitted to thesensor chip 411 by the connecting rod 417.

[0210]FIG. 21 is a view of the relationship between the sensor chip 411and the external force application plate 434 and the buffering mechanismas seen from below. In FIG. 21, to make the figure clearer, the plinthpart 412 is not shown. FIG. 22 is a view of the relationship between thesensor chip 411 and the buffering mechanism as seen from above. In FIG.22 the external force application plate 434 is not shown. The supportrods 431 and the middle support rods 433 of the buffering mechanism arecylindrical pillarlike members. The connecting rod 417 is also acylindrical pillarlike member, and is connected to the action part 421of the sensor chip 411 by an adhesive. To effect electrical separationfrom the sensor chip 411, an insulating member is interposed between thesensor chip 411 and the plinth part 412.

[0211] With a six-axis force sensor having this construction, becausethe buffering mechanism is made with a beam structure, the detectionsensitivities of Fz, Mx and My can be increased.

[0212] For a six-axis force sensor having the buffering mechanism shownin FIG. 20, when the relationship between the six axis components andthe output signals Sig1-Sig6 was obtained by the same method as in thecase of the sensor chip 411 described above, it was as follows.$\begin{matrix}{\begin{pmatrix}{{Fx}\quad\lbrack N\rbrack} \\{{Fy}\quad\lbrack N\rbrack} \\{{Fz}\quad\lbrack N\rbrack} \\{M\quad {x\quad\left\lbrack {N \cdot {cm}} \right\rbrack}} \\{{My}\quad\left\lbrack {N \cdot {cm}} \right\rbrack} \\{{Mz}\quad\left\lbrack {N \cdot {cm}} \right\rbrack}\end{pmatrix} = {\begin{pmatrix}128.9 & 0 & {.0} & 0 & 3.1 & 0 \\0 & 128.9 & 0 & {- 3.1} & 0 & 0 \\0.3 & 0.2 & 83.3 & {- 0.6} & {- 0.6} & 24 \\0 & 0.6 & 0 & 13.9 & 0 & 0 \\{- 0.6} & 0 & 0 & 0 & 13.9 & 0 \\0 & 0 & 0 & 0 & 0 & 60.6\end{pmatrix} \times \begin{pmatrix}{Sig1} \\{Sig2} \\{Sig3} \\{Sig4} \\{Sig5} \\{Sig6}\end{pmatrix}}} & (23)\end{matrix}$

[0213] By providing the sensor chip 411 with the box frame (bufferingmechanism) shown in FIG. 20, it is possible to attenuate the externalforce and reduce the strain arising in the sensor chip 411. Accordingly,the coefficients are larger than in the case of the sensor chip on itsown. When these coefficients are taken as the matrix elements of theabove equation, as in the matrix for the sensor chip on its own, thevalues of the diagonal elements are large and the non-diagonal elementsare either much smaller than the diagonal elements or are zero (“0”),and therefore the effect of suppressing other axis interference has beenmaintained. Looking at the diagonal elements, for Fx and Fy thecoefficients have increased by a factor of ten, and the detectableexternal force has increased by approximately ten times. Also for themoments Mx and My, the coefficients have increased by a factor of onehundred, and the range of detectable external force has also increasedby approximately one hundred times. Because it is possible to controlthe sizes of the coefficients by optimizing the structure of thebuffering mechanism, it is possible to realize desired characteristicsof the six-axis force sensor (for example, increase the withstandableforce Fz load).

[0214] FIGS. 23(a) and (b) show a comparison of an example measuringcharacteristic (a) of the six-axis force sensor pertaining to the firstspecific example shown in FIG. 20 and an example measuringcharacteristic (b) of a sensor chip on its own. In these figures, thehorizontal axis of each graph shows applied load Fz[N] and the verticalaxis shows resistance change proportion. As is clear from FIGS. 23(a)and (b), compared to the case of a sensor chip on its own, the ratedload of a six-axis force sensor according to this preferred embodimentis ten times larger. Thus the measurement range of the six-axis forcesensor has been raised by about ten times, and the load-withstandingperformance has increased.

[0215] A second specific example of a six-axis force sensor according tothe third preferred embodiment will now be described, with reference toFIG. 24. In FIG. 24, elements essentially the same as elements describedabove with reference to the first specific example have been assignedthe same reference numbers. In the six-axis force sensor of thisspecific example, four coil springs 441 are used as the bufferingmechanism. Rods 442 are attached to the four corners of the underside ofa square external force application plate 434, and the coil springs 441are fixed between these rods 442 and rods 431 fixed to the four cornersof the plinth part 412. The rods 431, 442 and the coil springs 441 formpillars having the function of buffering mechanism parts. The rest ofthe construction is the same as the first specific example. The six-axisforce sensor of this second specific example has the same actions andeffects as the first specific example.

[0216] A third specific example of a six-axis force sensor according tothe third preferred embodiment will now be described, with reference toFIG. 25. In FIG. 25, elements essentially the same as elements describedabove with reference to the first specific example have been assignedthe same reference numbers. In the six-axis force sensor of this thirdspecific example, a disc-shaped external force application part 451 isdisposed above the sensor chip 411, and this external force applicationpart 451 is supported in each of four places by two connected bar-likemembers 452, 453 which are respectively vertical and horizontal. Thefour bar-like member units form a buffering mechanism part. Thisconstruction has the characteristic that the horizontal bar-like members453 constitute beams in the same plane as that in which the externalforce application part 451 is disposed. The rest of the construction isthe same as that of the first specific example. The six-axis forcesensor of this third specific example has the same actions and effectsas the first specific example.

[0217] A fourth specific example of a six-axis force sensor according tothe third preferred embodiment will now be described, with reference toFIG. 26. In FIG. 26, elements essentially the same as elements describedabove with reference to the first specific example have been assignedthe same reference numbers. In the six-axis force sensor of this fourthspecific example, a disc-shaped external force application part 451 isdisposed above the sensor chip 411, pillars 461 are provided at the fourcorners of a square plinth part 412, supporting frame pieces 462 extendbetween the four pillars 461, and four supporting frame pieces 463 areprovided between the four supporting frame pieces 462 and the externalforce application part 451 to support the external force applicationpart 451. The supporting frame pieces 462 and 463 constitute beams whichform a buffering mechanism part. The rest of the construction is thesame as that of the first specific example. This construction has thecharacteristic D that beams are formed by the horizontal supportingframes 462, 463 in the same plane as that in which the external forceapplication part 451 is disposed. While six-axis force sensor of thisfourth specific example has the same actions and effects as the firstspecific example.

[0218] The present disclosure relates to the subject matters of JapanesePatent Application No. 2002-005334, filed Jan. 11, 2002, and JapanesePatent Application No. 2002-059447, filed Mar. 5, 2002, the disclosuresof which are expressly incorporated herein by reference in theirentireties.

What is claimed is:
 1. A six-axis force sensor comprising a thinplate-shaped sensor chip formed using a substrate by semiconductorfilm-forming processes and having a six-axis force sensor function, thesensor chip comprising an action part to which an external force isapplied, a support part to be fixed to an external structure, and aplurality of connecting parts each connecting together the action partand the support part and having a rigid bridge part joined to the actionpart and an elastic part joined to the support part, wherein each of themultiple connecting parts has a plurality of strain resistance deviceseach comprising an active layer formed on at least one of front and rearfaces thereof in an area thereof where deformation strain effectivelyarises and each electrically connected to corresponding electrodesdisposed in the support part.
 2. A six-axis force sensor according toclaim 1, wherein the connecting parts are disposed around the actionpart with uniform spacing and so that adjacent connecting parts aremutually perpendicular.
 3. A six-axis force sensor according to claim 1,wherein the action part is square and each of the connecting parts isformed in a T-shape made by its elastic part and its bridge part and isdisposed at a respective one of the four sides of the action part.
 4. Asix-axis force sensor according to claim 1, wherein the strainresistance devices are disposed on the surface of the bridge part of therespective connecting part near the boundary between the bridge part andthe action part.
 5. A six-axis force sensor according to claim 1,wherein the strain resistance devices are disposed on a narrowed portionformed in the bridge part of the respective connecting part.
 6. Asix-axis force sensor according to claim 1, wherein the strainresistance devices are disposed on the bridge part of the respectiveconnecting part in parallel with the length direction of the bridge partand arranged side by side in a line in a direction perpendicular to thelength direction of the bridge part.
 7. A six-axis force sensoraccording to claim 1, wherein a resistance device for temperaturecompensation is provided on the support part.
 8. A six-axis force sensoraccording to claim 1, wherein a guard interconnection at a groundpotential is provided so as to surround a non-ground interconnection ofa strain resistance device.
 9. A six-axis force sensor according toclaim 1, wherein a bias electrode for applying a bias potential isformed on the substrate.
 10. A six-axis force sensor according to claim1, wherein at least one of the strain resistance devices is disposed onthe rear side of the substrate in an area corresponding to an area onthe front side of the substrate where strain resistance devices aredisposed on one of the bridge parts.
 11. A six-axis force sensorcomprising: a thin plate-shaped sensor chip formed using a substrate bysemiconductor film-forming processes and having a six-axis force sensorfunction and comprising at least an action part for receiving anexternal force and a support part supporting the action part; and astructural body provided around the sensor chip and comprising anexternal force application part to which an external force is applied, aplinth part for supporting the sensor chip, an external force bufferingmechanism fixing the external force application part to the plinth part,and an external force transmitting part, wherein the external forceapplication part and the action part are linked by the external forcetransmitting mechanism.
 12. A six-axis force sensor according to claim11, wherein the sensor chip further comprises four connecting partsconnecting together the action part and the support part and a pluralityof strain resistance devices fabricated by semiconductor film-formingprocesses on deformable parts of the substrate.
 13. A six-axis forcesensor according to claim 11 or 12, wherein an insulating member isprovided between the sensor chip and the plinth part.